Systems of this kind can be used in modern telephone networks to allow authorized subscribers to have access to particular facilities, such as speech electronic mail, bank information, etc.. If the information obtainable by these facilities is confidential, the subscriber may have access to it only by pronouncing or dialing (or keying) his identification number, composed of a fixed number of digits, which can represent a word chosen out of a code. For various reasons, including line noises, symbol substitutions, deletions and insertions are possible and must be corrected during decoding.
In digital transmissions, it is common practice to use codes requiring transmission of sequences composed of a fixed number of symbols and allowing error correction by decoders at the receiving end, e.g., by exploiting redundancy symbols added to the information symbols and suitably computed: Reed-Soloman code is an example of such codes and characteristics are described, e.g. in the paper "A Universal Reed-Soloman Decoder", by R. Blahut, IBM Journal of Research and Development, Vol. 28, N. Mar. 2, 1984.
However, in digital transmissions, locations of possibly lost symbols can be identified by comparison with the noise level and symbol insertion is impossible (with the exception of catastrophic failures in timing systems). Hence known decoders are designed only to correct substitutions and fill erasures.
By contrast, for certain applications, including those envisages for the present invention, locations of lost symbols are not recognizable and, in addition, symbol insertions are possible. Hence known decoders cannot be used.